The present invention relates to an ink jet recording apparatus that prints images by utilizing variable pressures, produced in a pressure generation chamber, to selectively eject ink droplets through nozzle orifices.
Various types of ink jet recording apparatuses, such as ink jet printers and ink jet plotters, include a recording head for ejecting ink droplets upon the receipt of a drive pulses. In such an apparatus, as the recording head is reciprocally moved in a main scanning direction, ink droplets are ejected and form images on a recording medium.
In order to both improve the image quality and to increase the recording speed, an ink jet recording apparatus employs a variable-dot recording method whereby a plurality of ink droplets, each of which differs in volume, is ejected from the identical nozzle orifices.
According to the variable-dot recording method, a drive signal is generated that corresponds to one of a plurality of drive pulses, each of which produces the ejection of a different volume of ink, that are arranged in a time series, thereby forcing the selection of an appropriate drive pulse that is thereafter supplied to a pressure generating element.
In this recording operation, the quantity of ink in a droplet that is to be ejected is determined in accordance with the image that is to be printed. For example, a large ink droplet (a large dot) is ejected when a portion of an image having a relatively dark tone is printed, whereas a small ink droplet (a micro dot) is ejected when a portion having a relatively light tone is printed and a middle sized ink droplet (a middle dot) is ejected when a portion having an intermediate tone is printed.
As a result, a reduction in the recording speed due to an excessive increase in the pixel density can be prevented, and tones for four values, large, middle, small and 0 (no ejection), can be provided for each pixel, making it possible to more quickly and more clearly record a high-quality image.
In addition, while the recording head is moved in the reverse direction, bidirectional recording, during which dots are formed between other dots that were recorded while the recording head was moved in the forward direction, is performed and the printing, in a short time period, of a high density image is thereby enabled.
When the variable dot recording process is to be performed, each drive pulse, selected in accordance with a drive signal, is optimized, in accordance with the volume of ink that is to be ejected, so that a bias level (a reference voltage), the shape of a waveform, and a drive voltage (the pitch) differ for each drive pulse.
Since the bias levels of the drive pulses must be matched in order for a drive signal to be generated, a method is proposed whereby, while a drive pulse having a high bias level is employed as a reference, a drive pulse having a low bias level is superimposed on the high bias level.
However, if only the drive pulse having the low bias level is superimposed on the drive pulse having the high bias level, the maximum potential of the drive signal will exceed the upper limit of a drive circuit.
It is, therefore, one objective of the present invention to provide an ink jet recording apparatus wherein a drive signal, consisting of a plurality of drive pulses whose bias levels differ, can be appropriately generated within a limited voltage level range.
In order to achieve the above object, there is provided an ink jet recording apparatus comprising:
a recording head reciprocately moving in a main scanning direction with regard to a recording medium, the recording head provided with:
a nozzle orifice from which an ink drop is ejected;
a pressure chamber communicated with the nozzle orifice; and
a pressure generating element for generating pressure change in ink in the pressure chamber;
a drive signal generator for generating a drive signal in which a plurality of drive pulses configured to drive the pressure generating element to eject an ink drop from the nozzle orifice, respectively, the drive signal including:
a first drive pulse configured to drive the pressure generating element to eject an ink drop from the nozzle orifice, and to have a reference bias level;
a second drive pulse configured to drive the pressure generating element to eject an ink drop from the nozzle orifice, and to have an individual bias level which is different from the reference bias level;
a ready waveform for varying a potential of the drive signal from the reference bias level to the individual bias level, which is arranged in the drive signal so as to precede to the second drive pulse; and
a recovery waveform for varying the potential of the drive signal from the individual bias level to the reference bias level, which is arranged in the drive signal so as to follow the second drive signal; and
a drive pulse selector for selectively supplying at least one of the drive pulses and the waveforms in the drive signal to the pressure generating element to eject an ink drop from the nozzle orifice,
wherein the drive pulse selector selects the second drive pulse together with the ready waveform and the recovery waveform.
[Means for Solving the Problems]
To achieve the above objective, according to a first aspect of the invention, an ink jet recording apparatus comprises:
a recording head, which is reciprocally movable in a main scanning direction and which includes pressure generation chambers, which communicate with nozzle orifices, and pressure generating elements, for the application of alternate pressures in the pressure generation chambers;
drive signal generator that, to eject ink droplets, generates a drive signal consisting of a plurality of drive pulses, arranged in a time series, and that is adjusted to a reference bias level; and
drive pulse selector for selecting from the drive signal, which is generated by the signal generator, one of the drive pulses, which is supplied to the pressure generating elements to eject ink droplets through the nozzle orifices,
wherein the drive signal generated by the drive signal generator consists of
a first drive pulse at the reference bias level,
a second drive pulse at an individual bias level differing from the reference bias level,
a reference waveform for changing a voltage from the reference bias level to the individual bias level, and
a recovery waveform for changing a voltage from the individual bias level to the reference bias level,
wherein a ready waveform precedes the second drive pulse, and the recovery waveform follows the second drive pulse, and
wherein the drive pulse selector selects both the ready waveform and the recovery waveform when choosing the second drive pulse.
The drive signal generated by the drive signal generator includes: a first drive pulse at the reference bias level that corresponds to the bias level of the drive signal; a second drive pulse at the individual bias level that differs from the reference bias level; a ready waveform that is used to change the voltage from the reference bias level to the individual bias level; and a recovery waveform that is used to change the voltage from the individual bias level to the reference bias level. The ready waveform precedes the second drive pulse, and the recovery waveform follows the second drive pulse. With the second drive pulse, the pulse selector selects both the ready waveform and the recovery waveform.
Therefore, since the ready waveform is supplied before the second drive pulse, the voltage has already been changed from the reference bias level to the individual bias level when the second drive pulse is supplied. Furthermore, the recovery waveform is supplied after the second drive pulse in order to return to the reference level the voltage that was changed to the individual bias level when the second drive pulse was supplied.
As a result, even when a plurality of drive pulses having different bias levels are included in a drive signal, the maximum voltage of the drive signal can be suppressed, and the drive signal can fall within the limited range described by the voltage level.
Preferably, the drive signal includes:
a forward drive signal in which the plural drive pulses are arranged in a predetermined order, which is generated during a forward scanning of the reciprocate movement performed with the recording head; and
a reverse drive signal in which the plural drive pulses are arranged in an order resulted by inverting the predetermined order, which is generated during a reverse scanning of the reciprocate movement performed with the recording head.
Here, a period extending from a trailing end of the ready waveform to a leading end of the second drive pulse in the forward drive signal is coincided with a period extending from a trailing end of the ready waveform to a leading end of the second drive pulse in the reverse drive signal.
Preferably, the ready waveform is arranged in a head portion of the drive signal.
Preferably, a period extending from a leading end of the ready waveform to a trailing end thereof is equal or greater than a Helmholtz resonance cycle of the pressure chamber.
Preferably, a period extending from a leading end of the recovery waveform to a trailing end thereof is equal or greater than a Helmholtz resonance cycle of the pressure chamber.
Preferably, the ready waveform and the recovery waveform have a voltage gradient which is insufficient to eject an ink drop from the nozzle orifice, respectively.
Preferably, the individual bias level is set to a ground voltage.
Preferably, the second drive pulse serves as a reference drive pulse having an ejection waveform element for ejecting an ink drop which provides a positional reference in a pixel region. The ejection element is a waveform that serves as one part of a drive pulse, and according to which piezoelectric vibrators are activated for the ejection of ink droplets.
Here, a period extending from a leading end of the forward drive signal to a trailing end thereof and a period extending from a leading end of the reverse drive signal to a trailing end thereof are correspond to an unit print cycle. The unit print cycle is coincided with a sum of a period extending from the leading end of the forward drive signal to a leading end of the ejection waveform element in the forward drive signal and a period extending from the leading end of the reverse drive signal to a leading end of the ejection waveform element in the reverse drive signal.
Preferably, an interval between adjacent drive pulses in the forward drive signal is coincided with an interval between adjacent drive pulses in the reverse drive signal.
Preferably, the pulse selector selectively supplies the ready waveform and the recovery waveform to form a vibrating waveform for vibrating a meniscus of ink in the nozzle orifice at a magnitude at which an ink droplet will not be ejected in order to prevent a non-ejection state that causes the viscosity of ink to increase. This operation is performed during printing cycles.